In the field of electronics, multilayer semiconductor structures are frequently used.
A particular example of such structures is a structure of the semiconductor-on-insulator (SeOI) type.
A structure of the SeOI type typically comprises from its base to its surface a supporting substrate, an electrically insulating layer and a thin semiconducting layer, called an active layer, in or on which electronic components are generally intended to be formed.
When the thin layer is silicon, the structure is designated by the term of SOL an acronym of “Silicon On Insulator.”
The electrically insulating layer is a dielectric material, notably an oxide of the material of the supporting substrate and/or of the thin layer. This layer is then usually designated by the term of BOX, acronym of “Buried Oxide.”
Recently, SOI structures having an extra-thin layer of silicon have been developed.
These structures are designated by the term of “FDSOI,” for “Fully Depleted SOI,” i.e., a totally depleted SOT.
By “ultra-thin” is meant a thickness of less than or equal to 50 nm, preferably less than or equal to 12 nm, which may even be reduced down to about 5 nm.
FDSOI structures are particularly advantageous for producing planar electronic components, for example, FDMOS (acronym of “Fully Depleted Metal Oxide Semiconductor”) transistors, for which the channel is formed in or on the thin layer of silicon.
Because of the extreme thinness of the thickness of the thin layer, the threshold voltage of the transistor (usually noted as Vt), which depends on this thickness, is very sensitive to thickness variations of the thin layer.
For such applications, optimum uniformity of the thin silicon layer is, therefore, sought so as to have minimum Vt variability from one transistor to the other.
Considering the small dimensions of these devices and their great proximity, it is necessary to measure the thickness variation between points very close to each other, for example, every 0.5 μm.
This involves, during the method for making the SOI, measuring the thickness of the thin silicon layer and of the electrically insulating layer, in different points of the surface of the SOI, in a wide range of spatial wavelengths, typically comprised between 0.5 μm and 300 mm.
Present measurement methods are based on optical measurements, notably ellipsometry or spectral reflectometry.
In both cases, these methods involve conducting a large number of measurements by illuminating the SOI with a light flux having several optical wavelengths, so as to not only measure the thickness of the silicon layer but also that of the buried oxide layer.
However, with these techniques, it is not possible to conduct measurements with spatial wavelengths as small as 0.5 μm.
Thus, an ellipsometer allows measurements to be conducted with spatial wavelengths greater than or equal to about 40 μm.
On the other hand, these measurements take a long time and slow the manufacturing cycle of the SOIs.
Moreover, measurements conducted by ellipsometry or reflectometry with a single optical wavelength would not give the possibility of determining with sufficient accuracy the thickness of the silicon layer, since, for a given optical wavelength, the measured thickness of the silicon layer depends on the thickness and on the nature of the underlying buried oxide layer.
An object of the invention is, therefore, to propose a method for measuring thickness variations in a layer of a multilayer semiconductor structure, which gives the possibility of conducting measurements in particular in the range of spatial wavelengths comprised between 0.5 and 40 μm, which is not accessible with the present measurement methods.
In particular, the method should allow measurements of thickness variations in the thin silicon layer of an FDSOI structure, with an accuracy at least equal to 0.1 nm.